Regulating device for the correction of image defects of an electron-optical system



3,197,635 E DEFECTS OF AN AL IMAG M July 27, 1965 A. c. VAN DORSTEN ET REGULATING DEVICE F THE CORRECTION OF E TRON-OPTICAL SYSTE 2 Sheets-Sheet 1 Filed June 25, 1962 INVENTOR ADRIANUS C.VAN DORSTEN ADIgIYANUS J.J.FRANKEN fi g If.

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3,197,635 REGULATING DEVICE FOR THE CORRECTION OF IMAGE DEFECTS OF AN J y 1965 A. c. VAN DORSTEN ETAL ELECTRON-OPTICAL SYSTEM Filed June 25, 1962 2 Sheets-Sheet 2 INVENTOR ADRIANUS C.VAN DORSTEN ADRIANUS 1.1. FRANKEN AGENT United States Patent 3,197,635 REGULATiNG DEVICE FOR THE C(DRREQTHGN 0F IMAGE EFECTd OF AN ELEQTRQN-QHAL SYSTEM Adrianus Cornelis van Dcrsten and Adrianna Jacobus Jozef Franhen, Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company, Inc, New Y rlr, N.Y., a corporation of Delaware Filed lane 25, 1962, Ser. No. 204312 Claims priority, application Netherlands, Italy 16, 1961, 266,398

in electron microscopes and similar apparatus which project images of an object arranged in a beam of electric charge carriers it is known to reduce an image defect caused by the electron lenses by the addition of astigmatism the value and direction of which may be adjusted. Use is made of a regulating device and the desired correcting fields are produced by supplying electric voltages or currents to a system of auxiliary electrodes or magnetic poles disposed symmetrically about the beam axis. Such a system comprising electric and/or magnetic poles of equal strength and opposite sign is added to at least one of the electron lenses. It is referred to as a stigmator.

The systems most commonly used comprise eight correcting elements of which opposite elements have equal polarity while elements disposed at angles of 90 relative to one another have opposite polarity. The value of the astigmatism added to the electron lens by the system is controlled by variation of all pole strengths in the same sense While the direction of the astigmatism is adjusted by varying the pole strengths of adjacent poles in opposite senses. A suitable regulating device hence preferably has an adjusting knob for the value and an adjusting knob for the direction of the correcting field which are separately operated. To adjust the correcting system to provide optimum effect, in an electrostatic stigmator the voltages set up at adjacent electrodes are varied in 0pposite senses (so that the astigmatism caused by the stigmator rotates) until the residual astigmatism is a minimum. In this case the directions of the added astigmatism and of the astigmatism caused by the image defect are the same. By proportional variation of the voltages set up at all electrodes the astigmatism may then be completely eliminated.

A known regulating device for this purpose comprises fixed resistors and variable resistors constituting bridge circuits to which an adjustable direct voltage is applied. The values of the resistors are chosen so that by simultaneous variation of the adjustment of the variable resistors the voltages set up between the regulating members and the ends of the said resistors are varied in a predetermined manner.

Such a variation has also been produced by converting the rotary motion of an adjusting knob into translatory motion of the control members of two potentiometers. In this case, only two variable voltages are produced, while for a system of eight correcting elements at least four voltages are requ'ed. This means that for controlling the direction of the correcting astigmatism two coupled regulating members are required.

Another known regulating device comprises a rotary switch the number of moving parts of which is equal to the number of electrode pairs of the stigmator to which they are connected. The switch contacts are connected to tappings on a resistor. A controllable direct voltage is applied to the resistor. For a reasonably accurate adjustment the voltage steps should not be excessive and hence a large number of contacts are required in the switch. This gives rise to difl'iculty in mounting the re- 3 (Ilaims.

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quired number of tappings on the resistor. It is true that the required number of tappings is only one half of the number of contacts of the switch, however, in oruer to enable optimum voltage distribution to be obtained along the circumference of the switch the required number of tappings is still fairly large.

The invention relates to a device for controlling the voltages for the elements of an electrostatic correcting system enabling any desired voltage distribution and a continuously variable adjustment of the stigmator to be obtained. According to the invention the device includes a regulating resistor comprising a number of equal resistance elements connected to an adjustable direct voltage, each element comprising a layer of resistance material provided on an insulating support, the cross-sectional area of the resistance material gradually decreasing to a minimum between the terminals so that the resistance measured between one of the terminals and a point moving from one terminal to the other varies according to a nonlinear function from the beginning.

Sliding contacts are provided between adjacent terminals. They are mutually shifted through one half of the length of a resistance element and adapted to be moved simultaneously over contact areas of the resistance elements. Each sliding contact is separately connected to at least one electrode of the correcting system. The resistance elements form' a single continuous member. In each section the potential gradient is opposite to that in the adjacent section.

The combined elements may be united to form a rectilinear structure. However, it is in better agreement with the purpose of the voltage regulation, which is to change the direction of rotation of the added astigmatism, to use a rotary control member than to use a translatory member, and therefore it is of advantage for the contact area to be circular and for the resistance layer to extend along this area as a circularly curved strip of varying width. This strip may be bounded at the inner or outer side by a circle concentric with the contact area or it may vary in width on both sides of the contact area.

in known devices comprising a rotary regulating member the required agreement between the angle of rotation of said member and the direction of the added astigmatism is obtained by sinusoidal variation of the voltages set up at the correcting elements with the angle of rotation. An exact consideration of the field-strengths occurring in the usual stigmators when such voltage variation is applied snows that on variation of the direction of the added astigmatism its value also varies in inverse proportion to the number of stigmator poles. This obviously is a disadvantage. if the resistance elements described herein eforc are used this disadvantage may be obviated in a simple manner.

The pattern of the resistance elements may without dilrlculty be chosen so that the voltages taken from the contact area and applied to the stigma or poles vary sinusoidally on displacement of the slide contacts. However, the voltage values obtained when the adjusting knob is positioned between zero and the positive and negative maximum values are too high. By removing a small amount of the resistance material the values of the voltages derived at the intermediate positions may be varied and given the correct value for each position of the adjusting knob.

In order that the invention may readily be carried into effect, embodiments thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which FEGURE 1 shows a linear embodiment and FIGURE 2 shows a rotary controller, While FIGURE 3 is a longitudinal sectional view of an electron microscope.

FIGURE 4 shows a rotary controller to be used in combination therewith.

The circuit arrangement for the production of the control voltage is provided with terminals 1 and 2 adapted to be connected to a direct-voltage source. The terminals are shunted by a resistor 3 the center 4 of which is connected to earth. The positive and negative terminals of the current supply source consequently are symmetrical with respect to earth.

Between the positive terminal 1 and the earthed point 4 and between the negative terminal 2 and said point 4 two potentiometers 5 and 6 are connected. The control voltage is derived with the aid of sliders 7 and 8 which are coupled to one another. The sliders move simultaneously and the resistors 5 and 6 are connected so that by moving the sliders 7 and 8 in one direction the voltage derived is increased while by moving them in the other direction it is decreased. This voltage is adjustable between a maximum value and zero.

The stigmator is shown schematically by eight electrodes 9 and 16. Opposite electrodes 9-13, lit-14, 11-15 and 12-16 are electrically connected to one another. The electrode pairs 9-13 and 11-15 form a four-terminal network and the electrode pairs 19-14 and 12-16 form another four-terminal network.

The regulating member for the electrode voltages com prises resistance elements 17, 18 and 19. Although said elements are shown separately they form a continuous surface of resistance material applied to an insulating support. In the figure lines dividing the elements indicate terminals 29, 21, 22 and 23 to which the voltage is applied. The voltage may be applied to metal strips extending throughout the height of the resistance layer. The voltage required is taken from the sliders 7 and 8 of the potentiometers 5 and 6 of which the slider 7 is electrically connected to the terminals 29 and 22 and the slider 8 is elecrically connected to the terminals 21 and 23 so that when the resistance layer is scanned from one end to the other alternately a positive and a negative potential exists. Four contacts 24, 25, 26 and 27 are adapted to be moved over a contact area 2.8. They move simultaneously and their mutual spacings remain constant. Each contact is spaced from the nearest contact by one half of the length of the resistance element. This arrangement supplies the voltages required for a correcting system comprising eight electrodes. Any other even number of electrodes involves a modification of the arrangement.

The resistance elements are bounded on one side by a straight edge 29 and on the opposite side by a curved edge 30 of suitably chosen curvature, the thicknes of the layer of resistance material being assumed to be constant. The resistance material may substantially consist of carbon to which substances may be added to provide the required coherence of the carbon particles. Compositions suitable for manufacturing resistance layers are commonly known.

One may start from a sinusoidal variation of the voltage along the contact area, since for this variation the resistance pattern may readily be determined. The voltages derived by the contacts and supplied to the electrodes of the correcting system produce the electric correcting field for the stigmator but the strength of this field does not remain constant on movement of the contacts. To obtain a constant field strength the curved shape of the boundary of the resistance elements has to be slightly altered, for example to the form indicated by a broken line 31. This operation is simply obtainable by removal of a small amount of resistance material, the correct shape being most readily found empirically.

The entire resistance layer comprises a number of equal portions enabling the voltage of each pair of electrodes to be varied between positive and negative values. In the embodiment shown in FIGURE 1 three elements are require-d for this purpose.

In the rotary regulating device shown in FIGURE 2 two elements are sutlicient. The two resistance elements 32 and 33 are curved into the form of semi-circles and engage one another at the ends. Terminals 34 and 35 are provided at the junctions. The arrangement for adjusting the voltage is the same as that shown in FIGURE 1. The resistance layer is provided on an insulating plate 36 which also supports a vertical pin 37 provided with a knob 38. The pin 37 is made of insulating material and has four contact springs 3?, 4t), 41 and 42. attached to it and which consequently are insulated from one another. The free ends of these springs slide over the resistance layer and are connected each to a current conductor 43, 44, 45 and 4-6 respectively connected to electrodes 9 to 16 respectively of the correcting system. Similarly to the arrangement shown in FIGURE 1 each conductor is connected to a pair of opposite electrodes. The four-terminal network comprising the electrodes 9, 13 and 11, 15 has no voltage applied to it with the adjustment shown while the full voltage is applied to the electrodes 10, 14 and 11, 16 of the second tour-terminal network.

FIGURE 3 is a longitudinal sectional view of an electron microscope provided with a device in accordance with the invention.

The outer wall of this microscope is a steel tube 47 the upper end of which encloses an electrode system 48 serving for the production of the electron beam which extends along the axis of the tube 47 through the entire microscope. The microscope has three magnetic lenses: a condenser 49, an objective 56 and a projection lens 51. The design and operation of these lenses are assumed to be known. At its lower end the tube is provided with a glass window coated with a layer 66 of fluorescent material.

A system of correcting electrodes 52 serving to eliminate the astigmatism of the objective as far as possible is interposed between the objective 5i} and the projection lens 51.

To enable voltages to be set up between the various electrodes of the correcting system 52- they are provided with electric leads passing through the wall 47 in an insulating manner. For the sake of clarity the connections are separately shown externally of the tube.

The arrangement as shown in FIG. 4 for regulating the electrode voltages is more elaborate than in the preceding examples. The controller comprises four resistance elements 53, 54, 55 and 56 and the spindle corresponding to that of FIGURE 2 has eight sliding contacts. Each sliding contact is connected to an electrode of the correcting system, opposite electrodes being connected to alined contacts which in principle have equal potentials. In the embodiment shown, however, one may depart from this principle in order to provide a correcting system also enabling symmetry deviations to be eliminated. If the stigmator electrodes are not accurately centered about the optical axis the entire image is laterally displaced and on variation of the adjustment of the stigmator the image on the fluorescent screen is shifted.

The arrangement of FIG. 4 has four voltage supply terminals 57, 58, 59 and 60 which are displaced with respect to each other along the circumference. Between the two opposed terminals 57 and 59 the resistance element of a potentiometer 61 and between two other terminals 58 and 60 the resistance element of a potentiometer 62 is connected. The potentials of opposite terminals may be mutually varied so that voltage diiierences result which produce a transverse field between opposite stigmator electrodes which solely deflects the electron beam. Adjustment is eifected with the aid of potentiometer 61 and 62 by displacement of the sliders 63 and 64 which are connected to the sliders 7 and S of the potentiometers 5 and 6. The value of the applied voltage is adjustable in the manner described hereinbefore with the aid of potentiometers 5 and 6. The tapping 4 on the resistor 3 may also be adapted to be displaced for correction purposes.

What is claimed is:

1. A regulating device for correcting an astigmatic electron optical system in an electron beam focusing apparatus in which charge carriers are accelerated with the aid of an electrostatic stigmator having a plurality of electrodes, the value and the direction of the additional astigmatism required for the correction of the image defect of the electron-optical system being separately adjusted by variable resistors, said device comprising a control resistor connected to a source of adjustable voltage through terminals and further comprising a plurality of equal resistance elements each consisting of a layer of resistance material on an insulated support, the crosssectional area of each layer gradually decreasing to a minimum between the voltage terminals so that the resistance measured between one of the terminals and a point moving from one terminal to the other varies according to a non-linear function from the beginning, sliding contacts simultaneously movable over a contact area on the resistance elements between adjacent terminals, said contacts being spaced from one another by one half of the length of a resistance element and being connected each to at least one electrode of the correcting system, the resistance elements constituting a continuous member, the potential gradient in each section being opposite'to the potential gradient in the adjoining sections.

2. An arrangement as claimed in claim 1, in which the contact surface forms at least part of a circle at the centre of which is disposed a rotary spindle to which the sliding contacts are attached so as to be insulated from one another.

3. An arrangement as claimed in claim 1 in which the number of sliding contacts is equal to that of the electrodes of the correcting system.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NELSON, Primary Examiner. 

1. A REGULATING DEVICE FOR CORRECTING AN ASTIGMATIC ELECTRON OPTICAL SYSTEM IN AN ELECTRON BEAM FOCUSSING APPARATUS IN WHICH CHARGE CARRIERS ARE ACCELERATED WITH THE AID OF AN ELECTROSTATIC STIGMATOR HAVING A PLURALITY OF ELECTRODES, THE VALUE AND THE DIRECTION OF THE ADDITIONAL ASTIGMATISM REQUIRED FOR THE CORRECTION OF THE IMAGE DEFECT OF THE ELECTRON-OPTICAL SYSTEM BEING SEPARATELY ADJUSTED BY VARIABLE RESISTORS, SAID ADVICE COMPRISING A CONTROL RESISTOR CONNECTED TO A SOURCE OF ADJUSTABLE VOLTAGE THROUGH TERMINALS AND FURTHER COMPRISING A PLURALITY OF EQUAL RESISTANCE ELEMENTS EACH CONSISTING OF A LAYER OF RESISTANCE MATERIAL ON AN INSULATED SUPPORT, THE CROSSSECTIONAL AREA OF EACH LAYER GRADUALLY DECREASING TO A MINIMUM BETWEEN THE VOLTAGE TERMINALS SO THAT THE RESISTANCE MEASURED BETWEEN ONE OF THE TERMINALS AND A POINT MOVING FROM ONE TERMINAL TO THE OTHER VARIES ACCORDING TO A NON-LINEAR FUNCTION FROM THE BEGINNING, SLIDING CONTACTS SIMULTANEOUSLY MOVABLE OVER A CONTACT AREA ON THE RESISTANCE ELEMENTS BETWEEN ADJACENT TERMINALS, SAID CONTACTS BEING SPACED FROM ONE ANOTHER BY ONE HALF OF THE LENGTH OF A RESISTANCE ELEMENT AND BEING CONNECTED EACH TO AT LEAST ONE ELECTRODE OF THE CORRECTING SYSTEM, THE RESISTANCE ELEMENTS CONSTITUTING A CONTINUOUS MEMBER, THE POTENTIAL GRADIENT IN EACH SECTION BEING OPPOSITE TO THE POTENTIAL GRADIENT IN THE ADJOINING SECTIONS. 